How does fuel metering unit work ?

The working principle of the fuel metering unit

So what exactly does the fuel metering unit do? The fuel metering unit is also called the fuel metering proportional valve. It is installed in the fuel inlet position of the high-pressure fuel pump, used to adjust the fuel supply and fuel pressure value, and is controlled by the ECU computer.

Installation location

The fuel metering unit is installed at the fuel intake position of the high-pressure fuel pump to control the fuel intake and ensure that the pressure of the common rail is within the normal range, which plays a key role. Also called proportional solenoid valve, ECU controls valve opening and controls the amount of oil entering the high-pressure oil pump to control the common rail pressure.

Working principle

Here we can simply understand the fuel metering unit as an electromagnetic switch with an electronic coil inside which controls the conduction of the oil path leading to the oil pump.

When the switch is not powered on, the inlet metering valve is under the spring force of the return spring, and the control oil hole on the control spool is in the leftmost fully open position, and the amount of oil supplied to the oil pump is the largest. On the contrary, the solenoid valve is powered on. When in the zero oil supply position, the oil supply to the oil pump should be zero.

When the fuel metering valve enters the energized state, after the solenoid coil is energized, the iron core will generate electromagnetic force. With the continuous increase of the drive current, the electromagnetic force also increases. The fuel metering valve core supports the spring force of the return spring under the action of electromagnetic force. , Make the control spool move to the right until the electromagnetic force and the spring force reach a new balance, so that the control oil hole of the control spool is maintained at a specific position.

The continuous movement of the control spool to the right brings about the continuous reduction of the cross-sectional area of the control oil hole, and the continuous reduction of the oil intake. When the current increases to a certain extent, under the action of electromagnetic force, the control spool continues to move to the right until the control oil hole section on the control spool is completely covered by the internal control of the fuel metering valve seat, because the control spool and the fuel metering valve are A set of precision parts, so the oil intake can be completely shut off.

Take the normally open solenoid valve as an example. When the control coil is not energized, the fuel metering proportional valve is turned on and can provide the fuel pump with the maximum flow of fuel. ECU uses pulse signals to change the cross-sectional area of the high-pressure oil pump to increase or decrease the oil volume.

Therefore, the main parameters for judging the fuel metering valve are trigger current, voltage, frequency, duty cycle, fuel metering proportional integral and so on. When the fuel metering unit control valve core oil hole size and the spring force of the return spring are constant, the fuel flow direction through the fuel metering valve depends on the electromagnetic force passing through the suction control valve core, and the size of the electromagnetic force and the driving pulse width depend on the passing The voltage, current, frequency and duty cycle of the electromagnetic coil.

Solenoid Testing

Solenoid valves generally operate at a frequency between 100hz and 200hz. The frequency is low and the pulsation of oil will cause instability. The evaluation rate is too high, and the spring is closed before it is sucked up, and then the solenoid valve does not work. The general test range is 160-190hz. The duty cycle can be adjusted from 1. The electronic valve starts to make a sound but does not work. Gradually increase the duty cycle, the current gradually increases, and the louder sound can be heard, and the electronic valve starts to move and start working. The duty cycle should not exceed 50%, the current is too large, it is easy to damage the solenoid valve.

Precautions

The fuel metering unit is a precision component. Improper maintenance or the use of inferior filter elements will often result in excessive moisture or impurities in the fuel, which will cause the fuel metering valve core to wear or jam, and cause the engine to fail to work normally.